A report of Rhizopus oryzae causing postharvest soft rot of apple fruit in China
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Postharvest soft rot was found on apples in a local market in Beijing, China. Based on the morphological characteristics and molecular analyses of the ITS rDNA regions and Elongation factor alfa-1 (EFl-α) gene, the causal fungus was identified as Rhizopus oryzae. This is the first report of Rhizopus-associated with soft rot on apple fruit in China.
KeywordsRhizopus oryzae soft rot Apple Postharvest rot ITS sequence Ef-α1gene
Purified fungal colonies from infected tissues were initially white and cottony, then became grey to blackish-grey due to mature sporangia on PDA within three days (Fig. 1b). Sporangiophores were mostly erect, unbranched, smooth walled, aseptae, subhyaline to brown, singly or in groups and came up from stolons in opposite direction to rhizoids, in sets of 3–5. Sporangia were globose to subglobose, mostly 35-210 μm in diameter, white at first then becoming black due to the production of conidia (Fig. 1e). Columella were mostly globose to sub-globose and 30-90 μm × 50–110 μm. Sporangiospores were abundant, pale greyish to brown, sub-globose or ovoid, angular, having striations, and 3.8- 10 μm × 3-5 μm (Fig. 1f).
The in vitro growth response to temperature is important to distinguish between Rhizopus stolonifer (syn. R. nigricans) which does not grow above 37 °C and R. oryzae which does grows at 40 °C (Kwon et al. 2014; Schipper and Stalpers 2003). In this study, growth rate was also determined by measuring the diameters of fungal colonies on PDA plates incubated at 40 °C. Rhizopus isolates from affected apples grew at 30 mm/day on PDA at 40 °C, which together with their morphological features is characteristic of R. oryzae. The fungus was identified as Rhizopus oryzae, based on the morphological characteristics and growth temperature (Lunn 1977; Liou et al. 2007; Kwon et al. 2014; Schipper and Stalpers 2003).
To prove Koch’s postulates, nine apples were artificially inoculated with isolate YC-IK4 using a wound infection method. A spores suspension (0.1 mL; 106 conidia/mL of sterile distilled water) was injected under the surface of each apple at one point using a sterile needle under aseptic conditions in a laminar flow cabinet. Three apples were inoculated with sterile distilled water as a control treatment. All inoculated apples were placed in a closed chamber at 80% RH and 30 °C. After 48 h of incubation, fungal rot symptoms, similar to those on affected fruit collected from the local markets, developed on the inoculated apples. Control apples remained asymptomatic. The causal fungus was reisolated from the artificially infected apples and was identified as R. oryzae.
Postharvest soft rot of apple, sweet potato, and banana fruit caused by R. oryzae (syn; R. arrhizus) have been reported in Korea and Saudi Arabia (Kwon et al. 2011, 2012a, b; Al-Dhabaan 2018). The pathogen has also been reported to be associated with postharvest root rot of Codonopsis lanceolata in Korea (Park et al. 2014). However, to our knowledge, this is a new record of R. oryzae as a postharvest pathogen causing soft rot on apple in China.
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